1. FIELD OF THE INVENTION
The present invention relates to an illumination device for use in a slit exposure system of an image reader, electrophotographic copying machine or the like.
2. DESCRIPTION OF THE PRIOR ART
Generally, an image reader, electrophotographic copying machine or the like employs a tube-type light source having a filament suspended centrally thereof, the light source being surrounded by a converging type reflecting mirror. In operation, light emitted from the light source is reflected and converged by the mirror to illuminate an original document on a document table, and image information of the document is obtained from the light reflected by the same. Therefore, for obtaining accurate document image information, it is essential that the object portion of the document (the portion to be illuminated) be illuminated stably with sufficient illuminance.
In the actual devices of the above-noted type; however, there tends to occur an inconvenience in that such stable and sufficient illuminance is unobtainable due to inaccuracy in positioning of the filament attributable to attaching error of the light source or assembly error of the entire illumination device. Such positioning inaccuracy of the filament or inadvertent displacement of the same from its designed proper position leads to disadvantageous fluctuations or variations in the illuminance and consequently in the amount of light applied to the object document portion.
Especially, the above-described problem is serious in the case of the image reader, where image pickup elements for receiving the reflected light from the illuminated document are arrayed one-dimensionally with each element having a light-receiving portion of only 7 .mu.m width, which width yet corresponds to as much as 0.1 mm width in the illuminated document portion. That is to say, even a slight illuminance fluctuation or variation at the illuminated document portion due to the displacement of a filament may considerably affect accuracy of the obtained image information.
A Japanese laid-open patent application No. 56-74274 discloses an illumination device having a main reflecting mirror constituted by a plurality of oval surfaces and an auxiliary reflecting mirror constituted by a single flat surface. However, this device suffers from the same problem as described above. That is, in the disclosed device, since luminous flux emitted from a light source disposed at a first focal point common to the respective oval surfaces thereof is converged at its second focal point, even a slight displacement of the light source from the common first focal point may result in instability of illuminance applied to the object document portion.
A further reference, Japanese published patent application No. 60-15052 discloses another type of illumination device having a reflecting mirror constituted by a plurality of flat surface portions extending in the direction of an axis of its light source. However, this device has the disadvantage of insufficient light amount applied to the document portion because the respective flat surface portions provide no converging effect to the light emitted from the light source.